This invention relates to measurement of the rotation of a corrective toric contact lens.
The characteristic of astigmatism as applied to the eye itself is typically corrected by an asymmetric prescription of the corrective lens, which is called a toric contact lens. If the amount of astigmatism is known, there are known ways of manufacturing the asymmetric lens in order to cause the focus to occur on the retina. Frequently, however, difficulty occurs in the accurate determination of the prescription for the eye of a particular person having astigmatism and wishing to wear contact lenses.
To aid in this determination, contact lens manufacturers are known to create tiny, almost invisible markings such as dots and/or lines on the lens, located at the edges of the convexly curved lens. These markings can be formed by known techniques. With placement of the lens on the cornea of the eye, a care giver or clinician, such as an optometrist, an oculist, or an ophthalmologist, estimates the amount of contact lens rotation on the eye, usually from the horizontal or from the vertical, and prescribes according to the effect the rotation has on the person's astigmatism, the prescription being for a corrective toric contact lens. Such estimates are too often not accurate enough, resulting in errors of under-correction or overcorrection in lens rotation.
There are two significant areas to address in correcting vision astigmatism, 1) the location in degrees of the astigmatism in the patient's spectral astigmatic prescription, and 2) the amount in degrees of rotation of the toric contact lens while on the eye. The examination lens of the care provider provides the spectral astigmatic prescription location between 0 and 180 degrees. When the degree of rotation of the contact lens is determined, it has to be added to or subtracted from the patient's spectacle prescription to determine the proper toric contact lens prescription. That is, if the contact lens rotation is clockwise, it is called "left add" or "LA." If the lens rotation is counterclockwise, it is called "right subtract" or "RS." This contact lens rotation is caused by eyelid pressure and gravitational pull and other eye conditions. To help prevent contact lens rotation, sometimes prism ballast is added to the lens, and/or or slab off is used.
Techniques and devices have been proposed for more accurately ascertaining the degree of lens rotation, but these are typically inaccurate. Disclosed in the inventor's prior application Ser. No. 08/898,623 entitled METHOD AND APPARATUS FOR MEASURING TORIC CONTACT LENS ROTATION, filed Jul. 22, 1997, is a unique development for determining this characteristic. This present invention provides a further development from that prior invention.